How can I provide a Web App that will save vector data from individual users when they sign in but not seen by anyone else?

I would like to create a web app that requires a sign in. I want people to be able to add vector data to a map on their individual accounts for planning purposes but I don't want that data to be seen by other users. Does anyone know how I could do this or know of a company that would provide the service at a reasonable rate to users?

We have similar requirements, so I'll describe our approach. Your requirements may be similar enough to use and/or adjust the approach below.

User story

As the Emergency Manager, I need to access confidential features managed by each business unit so that I can make decisions based on all confidential features that the organization has. Managers in the business units choose whether or not to allow staff in other business units to see the confidential features they manage.

Solution idea (Portal or ArcGIS Online)

Each manager:

creates a group and adds their staff and the Emergency Manager to it

creates a hosted feature layer; enables editing, tracking, and per user editing as desired; and shares the hosted feature layer to their group

each manager creates a web app that uses the web map below and enables the desired capabilities to support the editing process.

The Emergency Manager:

creates a web map and adds all business unit hosted feature layers to it. This gives the Emergency Manager access to all of the confidential features.

shares that web map back to all of the business unit groups (if useful to the managers). This allows each group to see only its confidential features on the commonly shared web map.

creates a web app that uses that web map and enables the desired capabilities to support her decision process.

The solution designer:

Notes that ArcGIS Online terms of service explicitly prohibit using some types of confidential information

tries out the solution idea and adjusts until it meets requirements or finds its limitations within the ArcGIS Online capabilities. Limitations will include non-existent capabilities and bugs in existing capabilities.